We propose to study cellular interactions during embryonic development and during spermatogenic differentiation by producing and analyzing mouse aggregation chimeras that contain a lethal T/t-complex genotype and a normal genotype. The experimental design provides a way of defining the precise t-genotype of adult chimeras by breeding analysis and GLO-1 enzymology. Quantitative estimates of the relative number of cells of each of the two genotypes in both adult and embroyo chimeras will be obtained by GPI-1 analysis. With respect to embryogenesis, the objectives are to determine which (if any) of the lethal t-haplotypes act as cell lethals to define the cell lineages that are affected by t-lethals, and to examine the morphology of cell interactions when normal and mutant cells are intermingled. With respect to spermatogenesis, the objective is to determine whether or not the male transmission ratio distortion produced by t-haplotypes has a meiotic basis.